The preparation of hydrocarbons from an H.sub.2 /CO mixture by contacting this mixture at elevated temperature and pressure with a catalyst, is known in the literature as the Fischer-Tropsch hydrocarbon synthesis process. The products that can be prepared usually possess a very wide molecular weight distribution and in addition to branched and unbranched paraffins often contain considerable quantities of olefins and oxygen-containing organic compounds. As a rule only a minor portion of the resultant products consists of so-called middle distillates. "Middle distillates" refers to hydrocarbon mixtures having a boiling temperature range principally corresponding to that of the kerosine and gas oil fractions which are obtained in the conventional atmospheric temperature range distillation of crude petroleum. The middle distillate temperature range lies substantially between about 150.degree. and 360.degree. C. Besides the yield, the pour point of these middle distillates is not optimal. Accordingly, the direct conversion of H.sub.2 /CO mixtures by the Fischer-Tropsch process is not a particularly attractive route for the preparation of middle distillates on the technical scale.
The British patent 2,161,177 discloses a similar hydrocarbon synthesis process, using Fischer-Tropsch catalysts. The yield of middle distillates, expressed in terms of C.sub.5.sup.+ selectivity, is improved if a catalyst having a specific texture is used, namely an external surface area of 5-70 cm.sup.2 /ml, an internal surface area (SI of 10-400 m.sup.2 /ml, under the provision that 10.sup.6 &gt;S.sub.e.sup.2 .times.S.sub.i &gt;2.5.times.10.sup.4.
The C.sub.5.sup.+ selectivity improvement is only disclosed for a hydrogen/carbon monoxide feed ratio (F) of 2.